BACKGROUND
Normal thyroid function during pregnancy is critically important for baby’s normal development. Too low thyroid hormone levels in mothers during pregnancy can cause complications in pregnancy and affect baby’s body and brain development. Pregnant women without thyroid disease naturally make more thyroid hormone during pregnancy to provide enough for both mother and baby. In women with preexisting hypothyroidism (low thyroid hormone levels) taking levothyroxine before pregnant, the levothyroxine dose generally needs to be increased during pregnancy, usually by 25-30%. Many pregnant patients may not know that they are pregnant until 1-2 missed menstrual periods and early pregnancy is a critical time for baby’s development. Therefore, it is important for patients with hypothyroidism planning pregnancy to have normal thyroid levels before becoming pregnant.

Generally, the thyroid stimulating hormone (TSH) level in blood is a good indication of thyroid status. TSH is opposite to thyroid hormone levels (i.e. high TSH level means low thyroid hormone levels and low TSH level means high thyroid hormone levels). The current American Thyroid Association guidelines recommend that women who are treated with levothyroxine for hypothyroidism and planning for pregnancy should have their TSH levels checked before becoming pregnant and to have their levothyroxine dose adjusted to keep TSH below 2.5mIU/L. This study assessed how many women with known hypothyroidism had TSH levels measured and how many of these women had normal thyroid levels within the year before pregnancy.

THE FULL ARTICLE TITLE
Tena Vivó G et al 2023 Description of thyroid disorders the year before conception: A population-based study. Front Endocrinol (Lausanne) 14:1236505. PMID: 37818089.

SUMMARY OF THE STUDY
A total of 120,763 pregnancies from medical records registry of primary care offices and hospitals of women living in Catalonia, Spain from 2014-2026 was included in the study. Thyroid function test results during the year before pregnancy were reviewed. About 2800 women (~2.4%) had hypothyroidism and were taking levothyroxine before pregnancy. Among these women with known thyroid disease, about 77% had a thyroid level measured in the year before pregnancy. Of these women with known hypothyroidism and available thyroid levels, 42% had TSH levels less than 2.5mIU/L (ideal) and 32% had TSH above normal range, indicating lower than recommended thyroid hormone levels.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
In a large group of pregnant women in Catalonia region of Spain, almost a quarter of women with history of hypothyroidism taking levothyroxine did not have thyroid level results available in the year before pregnancy. Among those who had TSH levels available, only 42% had TSH level in a good range for pregnancy. These findings are an important reminder that monitoring and care of women with hypothyroidism planning pregnancy can be much improved. Clinicians as well as patients should be aware of the importance of normal thyroid function and current recommendation regarding thyroid function monitoring before pregnancy.

— Sun Y. Lee, MD, MSc

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